Stepper motor homing method and system

ABSTRACT

A method or system for bringing a movable element of a stepper motor to its home position by mechanically blocking the movable element from further moving from the home position, wherein the movable element is moved in one direction by repeating an energization cycle in which windings for sequentially establishing different energization phases are energized in a predetermined sequence at a predetermined interval from one phase to another. After the movable element is mechanically blocked, a motor driver circuit energizes at least one of the windings which corresponds to at least one of the energization phases that precedes another of the energization phases which corresponds to the home position of the movable element. This energization of the winding or windings for each preceding phase continues for a time longer than the predetermined energization interval of the normal energization cycle. Then, the phase corresponding to the home position of the movable element is maintained to hold the motor stopped at the predetermined home position.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to a technique for bringing astepping motor to a predetermined home position, and more particularlyto such a technique wherein a precise homing control of the steppermotor is effected after a movable element of the motor such as a rotoror a linearly sliding member is mechanically blocked at the homeposition.

2. Discussion of the Prior Art

U.S. Pat. No. 4,264,220 discloses an apparatus for homing a print wheelof a typewriter, wherein the print wheel is brought to its predeterminedhome position by mechanically blocking the print wheel at the homeposition, such that a first stop member provided on a member forrotating the print wheel comes into engagement with a fixed second stopmember. According to the disclosed arrangement, the stepper motor isheld in an energization phase corresponding to the home position, afterthe print wheel is mechanically blocked by the second stop member. Thus,the stepper motor is maintained at its home position. Thus, arrangementeliminates a detector for sensing the home position of the print wheel,and contributes to lowering the cost of the typewriter.

The homing method proposed in the above-identified U.S. Patent is widelyapplicable to a variety of devices or systems wherein a movable memberdriven by a stepper motor need to be zeroed or placed in its homeposition. The method is applicable not only to a rotary stepper motor,but also to a linear stepper motor.

Practically, however, the stepper motor cannot be precisely stopped atthe predetermined home position simply by mechanically blocking themovable member. The motor can be brought exactly to the home position ifit is possible to hold the energization phase corresponding to the homeposition of the motor, exactly at the moment when the movable element ofthe motor has been mechanically stopped by engagement of the first andsecond stop members. In reality, it is difficult to control the timingof energization of the windings in precise synchronization of themechanical blocking of the print wheel. Usually, the energization phasesare changed from one to another for a given time after the movableelement has been mechanically stopped. Thus, the stepper motor may failto be stopped at the predetermined home position.

For example, a rotary stepper motor is operated in a simultaneous twophase energizing mode wherein different combinations of four windings A,B, C and D are energized in a predetermined sequence so as tosequentially establish phases A-B, B-C, C-D and D-A, as indicated inFIG. 9. Suppose the phase C-D of the motor corresponds to the homeposition of the motor at which the rotor of the motor is mechanicallystopped during rotation in the forward direction, the rotor may remainin the home phase or phase C-D position during the next energization ofthe windings D and A to establish the phase D-A. Alternatively, therotor may be rotated toward the phase D-A position during the phase D-Aenergization, even after the rotor is mechanically blocked around thephase C-D position, since there exists a cushioning action of the secondstop member upon abutting contact of the first stop member with thesecond stop member. The cushioning action permits the rotor to befurther rotated by a slight angle in the forward direction, and thusallows the rotor to be moved toward the phase D-A position. However, inthe next energization of the windings A and B so as to establish thephase A-B, the rotor teeth can not be moved further in the forwarddirection, and tend to be rotated in the reverse direction as indicatedin FIG. 9, while being attracted by the wrong stator teeth of the samephase A-B neighbouring the true phase A-B stator teeth to which therotor teeth should be attracted in that energization step. Subsequently,the rotor is rotated again in the forward direction as the phases B-Cand C-D are established. The above events are repeated, and thus therotor undergoes oscillating movements to and from the home phaseposition (phase C-D position) over a given angular range while theenergization cycle is repeated, as indicated in solid line in thefigure. During such oscillating movements of the rotor, the rotor mayjump to the wrong phase C-D position which neighbours the true phase C-Dposition corresponding to the home position of the rotor, as indicatedin broken line in the figure. In this instance, the rotor is erronesoulystopped at the wrong phase C-D position indicated in the broken line.Similar drawbacks are encountered also in a linear stepper motor. In thelinear stepper motor, the movable element is a linearly moving membercorresponding to the rotor of the rotary stepper motor.

SUMMARY OF THE INVENTION

It is accordingly an object of the present invention to provide a methodby which it is possible to bring the movable element of a stepper motorexactly to its predetermined home position after the movable element ismechanically stopped.

Another object of the invention is to provide an apparatus using astepper motor, wherein the movable element of the motor is broughtexactly to its predetermined home position after the movable element ismechanically stopped.

It is a further object of the present invention to provide a printerhaving a print wheel which is operated by a rotary stepper motor,wherein the print wheel is brought exactly to its predetermined homeposition after the rotor of the motor or the print wheel is mechanicallystopped.

According to the present invention, there is provided a method ofbringing the movable element of the stepper motor to a predeterminedhome position, the method comprising the steps of: moving the movablemember in one direction by repeating an energization cycle whereinwindings for sequentially establishing a plurality of energizationphases of the motor are energized in a predetermined sequence at apredetermined energization interval from one energization phase toanother; mechanically blocking the movable element from further movingfrom the home position in the above one direction; after mechanicallyblocking the movable element, energizing at least one of the windingswhich corresponds to at least one of the energization phases thatprecedes, in the above-indicated predetermined sequence, another of theenergization phases which corresponds to the predetermined home positionof the movable element, the above-indicated at least one of the windingsbeing energized for a time span longer than the predeterminedenergization interval of the energization cycle, for each of the atleast one energization phase preceding the another energization phase;and maintaining the above-indicated another energization phasecorresponding to the home position of the movable element, and therebyholding the movable element at the predetermined home position.

According to the above method of the invention, the movable element ofthe stepper motor can be stopped exactly at the predetermined homeposition, without fail.

For instance, when the winding or windings are energized for theabove-indicated relatively long time span so as to establish theenergization phase corresponding to the home position of the movableelement, the movable element oscillates in an initial portion of theenergization time span, and is eventually stopped at the predeterminedhome position, or alternatively at a position of the wrong stator teethof the same energization phase which neighbours in the reverse directionthe true stator teeth corresponding to the home position. Even in thelatter case, the movable element can be brought to the home position,upon completion of at least one energization cycle which takes placeafter the mechanical blocking of the movable element. In this condition,the phase corresponding to the home position is held energized, wherebythe movable element is held in the home position. In the former casewherein the movable element is located at its home position at the endof the first long energization, the movable element is once stepped awayfrom the home position in the reverse direction due to the second orsubsequent energization. However, the movable element is finally broughtto the home position by the following stepping movement or movements inthe forward direction due to the subsequent energization step or steps.

Thus, the movable element can be always brought to the predeterminedhome position, under any conditions, by energizing the windingscorresponding to at least one energization cycle so as to sequentiallyestablish the different phases one afte another at the relatively longenergization interval after the movable element is mechanically blocked.However, if the amplitude of the oscillating movement of the movableelement following the mechanical blocking is short, and the movableelement tends to be moved to the phase position just before the phaseposition corresponding to the home position, the movable element can bebrought to the home position by energizing only the winding or windingcorresponding to that preceding phase, for the relatively long timespan, after the movable element is mechanically blocked, and before thewinding or windings corresponding to the home position are energized. Inthis case, the home position of the movable element is established byonly one stepping movement of the movable element which results from thesingle operation to energize the appropriate winding or windings for thetime span longer than that of the normal energization cycle.

The amplitude of the oscillating movement of the movable element whichoccurs following the mechanical blocking is determined by the moment ofinertia and other parameters of a device or component which is driven bythe stepping motor. Hence, the required minimum number of steps ofenergization each for the relatively long time span is determined by thespecific device or component driven by the stepper motor.

According to the instant method, the movable element of the steppermotor can be stopped precisely at the predetermined home position,without using a detector for detecting the home position of the movableelement or a home position of a member driven by the stepper motor.Further, the method according to the invention eliminates aconventionally experienced inaccurate homing of the movable elementwhich arises from erroneous electromagnetic attraction of the movableelement, for example, by a pair of energized stator pole pieces whichare located adjacent to the stator pole pieces assigned to establish thephase corresponding to the home position of the movable element.

The above advantages can be offered without substantially increasing thecost of the apparatus incorporating the stepper motor, since the homingof the movable element of the motor simply requires energizing at leastone of the windings of the motor for the phase that precedes the phasecorresponding to the home position of the movable element, after themovable element has been mechanically blocked.

According to another aspect of the present invention, there is providedan apparatus which comprises: (a) a stepping motor including windingsfor sequentially establishing a plurality of energization phases, and amovable element which is movable in opposite directions; (b) a motordriver circuit connected to the windings, for moving the movable elementin one of the opposite directions, by repeating an energization cycle inwhich the windings are energized in a predetermined sequence at apredetermined energization interval from one energization phase toanother; (c) a mechanical blocking means for mechanically blocking themovable element from further moving away from a predetermined homeposition in the above-indicated one direction; and (d) homing controlmeans operable after the movable element has been mechanically blockedby the mechanical blocking means, for activating the motor drivercircuit to energize at least one of the windings which corresponds to atleast one of the energization phases that precedes, in the predeterminedsequence, another of the energization phases which corresponds to thepredetermined home position of the movable element. The above-indicatedat least one of the windings is energized for a time span longer thanthe predetermined energization interval of the energization cycle, foreach of the at least one energization phase preceding theabove-indicated another energization phase. The homing control meansthen activates the motor driver circuit to maintain the above anotherenergization phase, and thereby hold the movable element at thepredetermined home position.

According to one advantageous feature of the above aspect of theinvention, the homing control means activates the motor drive circuit toenergize the windings in the predetermined sequence, in order to performat least one energization cycle of the motor.

In accordance with a further aspect of the invention, there is provideda printer for printing on a recording medium, comprising: (a) supportmeans for supporting the recording medium; (b) a print wheel rotatableabout an axis thereof, and having a multiplicity of radial arms whichbear at free ends thereof respective typing elements; (c) a rotarystepping motor including a rotor rotatable in opposite directions andoperatively connected to the print wheel, and windings for sequentiallyestablishing a plurality of energization phases; (d) a motor drivercircuit connected to the windings, for repeatedly performing anenergization cycle in which the windings are energized in apredetermined sequence at a predetermined energization interval from oneenergization phase to another, whereby the rotor is rotated to bringselected one of the typing elements into a printing position; (e) ahammer operable to impact the selected one typing element in theprinting position against the recording medium supported by the supportmeans; (f) a mechanical blocking means for mechanically blocking theprint wheel from further rotating from a predetermined home position inone of the opposite directions; and (g) homing control means operableafter the movable element has been mechanically blocked by themechanical blocking means, for activating the motor driver circuit toenergize at least one of the windings which corresponds to at least oneof the energization phases that precedes, in the predetermined sequence,another of the energization phases which corresponds to thepredetermined home position of the movable element. The above-indicatedat least one of the windings is energized for a time span longer thanthe predetermined energization interval of the energization cycle, foreach of the above at least one energization phase preceding the anotherenergization phase. The homing control means then causes the motordriver circuit to maintain the above-indicated another energizationphase, and thereby hold the rotor at the predetermined home position.

According to one advantageous feature of the above aspect of theinvention, the time span for which the above-indicated at least onewinding is energized for each of the at least oen energization phase islonger than a time duration necessary for substantially settling arotary oscillating movement of the print wheel during a mechanicalblocking of the print wheel by the mechanical blocking means.

According to another advantageous feature of the same aspect of theinvention, the printer further comprises a carriage carrying thereon thestepping motor and the print wheel, and movable in a direction parallelto the recording medium supported by the support means, and furthercomprises a wheel holder connected to the rotor of the stepping motorrotatably about an axis thereof, and holding the print wheel. In thiscase, the mechanical blocking means includes: a detent arm attached tothe carriage pivotally about an axis parallel to the axis of rotation ofthe wheel holder, the detent arm having a first stop member; a secondstop member provided on the wheel holder, such that the second stopmember is engageable with the first stop member during rotation of thewheel holder, for mechanically blocking the wheel holder, and therebyblocking the print wheel from further rotating away from thepredetermined home position; and an arm actuator for pivotally movingthe detent arm between an operative position in which the first stopmember is engageable with the second stop member, and an inoperativeposition wherein the first stop member is out of a path of the secondstop member.

In one form of the above feature of the invention, the arm actuatorcomprises biasing means for biasing the detent arm toward theinoperative position, and a stationary actuator member which isengageable with the detent arm, to pivot the detent arm to the operativeposition against the biasing action of the biasing means, when thecarriage is moved to a predetermined position.

In another form of the same feature of the invention, the detent arm isprovided with a resilient member fixed thereto. The resilient member isadapted to be elastically deformable to permit the carriage to move tothe predetermined position, even if the wheel holder is positioned suchthat the second stop member abuts on the first stop member during apivotal movement of the detent arm toward the operative position.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the presentinvention will be better understood by reading the following detaileddescription of preferred embodiments of the invention, when consideredin connection with the accompanying drawings, in which:

FIG. 1 is a fragmentary side elevational view in cross section of aprinting mechanism of one embodiment of a printer of the presentinvention, in the form of a typewriter having a stepping motor which isbrought to its home position according to the invention;

FIG. 2 is a front elevationary view of the printing mechanism shown inFIG. 1;

FIG. 3 is a schematic fragmentary view of the interior construction ofthe stepping motor;

FIG. 4 is a block schematic diagram illustrating a control system forcontrolling the stepper motor of the typewriter;

FIG. 5 is a flow chart representing an example of a control program forbringing the stepper motor to its home position, in one form of a methodaccording to the invention;

FIG. 6 is a diagram indicating movements of the rotor of the steppermotor, in relation to different energization phases;

FIGS. 7 and 8 are diagrams corresponding to that of FIG. 5, depictingmodified embodiments of the method of the invention; and

FIG. 9 is a diagram also corresponding to that of FIG. 5, showing therotor movements controlled according to a conventional arrangement.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring first to FIGS. 1 and 2, there is shown a printing mechanism ofa typewriter to which the principle of the present invention is appliedto establish a home position of a rotary stepper motor 20 for operatinga print wheel 28. In the figures, reference numeral 10 designates acarriage which is moved parallel to a platen 16 while being supportedand guided by a guide rod 12 and a guide rail 14. The platen 16functions as support means for supporting a recording medium in the formof a sheet of paper 46 on which printing is effected along a print lineparallel to the axis of rotation of the platen.

The carriage 10 has a motor bracket 18 fixed thereto, and carries thestepper motor 20 attached thereto via the motor bracket 18. As shown inFIG. 3, the stepper motor 20 has a rotor 19 which is rotatable inopposite directions by sequential energization of stator windings A, B,C and D, as well known in the art. These windings A, B, C and D arewound on respective stator pole pieces 21 which are arranged inspaced-apart relation in the circumferential direction of the rotor 19.The stator pole pieces 21 have stator teeth 21a which are formed so asto face rotor teeth 19a formed on the rotor 19. The operation of thestepper motor 20 will be described later in detail.

The output shaft of the stepper motor 20 fixed to the rotor 19 has apinion 22 fixed thereto, and is connected to the print wheel 28 suchthat the pinion 22 meshes with a gear 24 which is rotatably supported bythe bracket 18 and is fixed to the print wheel 28. The gear 28 has ashaft 26 which extends through the bracket 18. The print wheel 28 issecured to the free end of the shaft 26, at a predetermined angularposition relative to the shaft 26. Thus, the shaft 26 functions as aholder for rotatably supporting the print wheel 28. The print wheel 28has a multiplicity of radial arms 30 which are spaced apart from eachother in the circumferential direction. The radial arms 30 has differenttyping elements 32 at their free ends. In this embodiment, a total of 96typing elements 32 are provided.

On the motor bracket 18, there is also fixed a pin 40 which pivotallysupports a hammer bracket 38, which in turn supports a hammer 42 fixedthereto. The hammer 42 is pivotally operated to its operating positionby a hammer solenoid 43 secured to the motor bracket 18. With the hammer42 pivoted to its opeating position, the currently selected typingelement 32 is struck or impacted at its back, against the paper 46 via aprint ribbon 44. Thus, a character corresponding to the hammered typingelement 32 is printed on the paper 46.

The motor bracket 18 has a shaft 50 fixed thereto, and a detent arm 52which is pivotally supported about the axis of the shaft 50. Morespecifically, the detent arm 52 is pivotable in a plane parallel to therotating plate of the gear 24, between an operative position and aninoperative position. The detent arm 54 has a first stop member in theform of a hook 54 formed integrally at its free end, while the gear 24has a second stop member in the form of a tab 56. While the detent arm52 is placed in its operative position, the hook 54 is engageable withthe tab 56 during rotation of the gear 24. The detent arm 52 is biasedby a spring 58 in a counterclockwise direction (as viewed in FIG. 2),whereby the detect arm 52 is normally placed in its inoperativeposition, which is determined by a stop 60.

The detent arm 52 is provided with a resilient member 62 securedthereto. The resilient member 62 comes into abutting contact with astationary arm actuator 64, when the carriage 10 has moved in the rightdirection (in FIG. 2) to a predetermined reference position within aright margin area beyond a preset printing zone. As a result of thisabutting contact, the detent arm 52 is pivoted in the clockwisedirection to the operative position, against the biasing action of thespring 58, for engagement of the hook 54 with the tab 56 on the gear 24.Thus, the detent arm 52 having the hook 54, and the tab 56 on the gear24, cooperate to function as means for mechanically blocking the gear 24and therefore the print wheel 28 from further rotating in the clockwisedirection (in FIG. 2) away from a predetermined home position. Further,the spring 60, resilient member 62 and stationary actuator 62 cooperatewith a drive source for the carriage 10, to serve as an actuator foroperating the detent arm 52 between its operative and inoperativepositions.

Referring next to FIG. 4, the stepping motor 20 is driven by a motordriver circuit 80, which is controlled by a microcomputer 78 thatincludes a central processing unit 70 (CPU), a read-only memory 72 (ROM)and a random-access memory 74 (RAM). The ROM 72 stores various controlprograms for controlling the typewriter, such as a program for effectinga motor homing routine (illustrated in the flow chart of FIG. 5) forbringing the stepper motor 20 to its predetermined home position. TheCPU 70 is adapted to execute these control programs, while utilizing atemporary data storage function of the RAM 74.

There will next be described the motor homing routine, by reference tothe flow chart of FIG. 5.

As soon as the carriage 10 has been moved to its reference position andthe detent arm 52 has been pivoted to its operative position formechanically blocking the gear 24, the reference position of thecarriage 10 is sensed by a suitable detector. A signal generated by thisdetector is applied to the CPU 70, and the motor homing routine of FIG.5 will be executed as described below.

Initially, the CPU 70 executes step S1 wherein a value "98" is set in acounter. This value "98" is the number of the typing elements 32 plus"2", that is, "96" plus "2" (98=96+2). Step S1 is followed by step S2wherein the stepper motor 20 is incremented one step to rotate the printwheel 28 one step in the forward direction (clockwise direction in FIG.2). For easy understanding, it is assumed in this embodiment that onestep of operation of the stepper motor 20 results in one indexing stepof the print wheel 28 (which causes the currently selected typingelement 32 to be replaced by the next one). Then, the CPU 70 goes tostep S3 to start or turn on a first timer which is set to measure a timeperiod of 3 msec. The completion of measurement of this time period instep S3 causes the CPU 70 to decrement the counter by one ("1") in stepS4. The CPU 70 then goes to step S5 to check if the counter has beenzeroed or not. At this point of time in the present specific example,the current count of the counter is "97", and therefore the controlreturns to step S2, to repeat steps S2-S5.

The above steps S2-S5 are repeated until the judgement in step S5becomes affirmative (YES), namely, until the stepper motor 20 has beenenergized 98 steps which correspond to one full revolution of the printwheel 28, plus a fraction of one revolution corresponding to the twoindexing increments (two typing elements 32) of the print wheel 28.

It will be understood that the tab 56 on the gear 24 comes intoengagement with the hook 54 of the detent arm 52 in the operativeposition, at a certain point of time during the 98 steps of energizationof the windings of the stepper motor 20. Namely, irrespective of thespecific angular position of the print wheel 28 when the carriage 10 hasbeen moved to its reference position, the print wheel 28 can bemechanically blocked at the predetermined home position throughengagement of the hook 54 and the tab 56, during the 98 steps ofenergization of the stepper motor 20. Even after the rotor 19 of thestepper motor 20 has been mechanically blocked by the mechanicalblocking of the gear 24, the windings A, B, C and D of the stepper motor20 are energized by the remaining number of steps in a predeterminedsequence, with the rotor 19 repeating a small oscillating movement toand from the blocked position, as indicated in FIG. 9.

If the gear 24 is positioned such that its tab 56 abuts on the hook 54of the detent arm 52 during a pivotal movement of the detent arm 52toward its operative position when the carriage 10 has approached thepredetermined reference position, the resilient member 62 is elasticallydeformed, permitting the carriage 10 to move to the predeterminedreference position, even in the above case. When the gear 24 issubsequently rotated, the tab 56 disengages from the hook 54, and thedetent arm 52 is pivotally moved to its operative position under thebiasing action of the resilient member 62. The tab 56 comes intoengagement with the hook 54 when the gear 24 has rotated a substantiallyone full revolution.

In the present embodiment, an energization cycle is performed in asimultaneous two-phase mode. Described in more detail, the energizationcycle occurs such that four different pairs of the four stator windingsA, B, C and D are energized in the predetermined sequence, to establishphases A-B, B-C, C-D and D-A in this order, as indicated in FIG. 6. Forexample, when the windings A and B are simultaneously energized, thestepper motor 20 is placed in the phase A-B in which the rotor 19 isplaced in a phase A-B position. As the winding pairs are sequentiallyenergized, the four different phases are sequentially established,whereby the rotor 19 is rotated. In this specific example, thepredetermined home position of the rotor 19 and the print wheel 28corresponds to the phase C-D of the energization cycle.

Referring back to step S5, if the decision in the step is affirmative,that is, if the windings of the stepper motor 20 have been energized 98steps, step S5 is followed by step S6 in which the CPU 70 checks to seeif the stepper motor 20 is placed in the phase C-D. If not, step S6 isfollowed by step S7 wherein an additional value "1" is set in thecounter previously indicated. Then, steps S3-S5 are performed toenergize the stepper motor 20 one more step. These steps S7 and S2-S6are repeated until an affirmative decision is obtained in step S6. Ifstep S6 gives an affirmative decision, the CPU 70 then goes to step S8wherein a value "4" is set in the counter. Step S8 is followed by stepS9 to turn on a second timer. This second timer is set to measure a timeperiod of 40 msec, in this specific example. Upon elapse of this settime of 40 msec., the stepper motor 20 is energized one step in stepS10, and the counter is decremented by one "1" in step S11. Then, stepS12 is executed to check if the counter has been zeroed or not. In thismanner, steps S8-S12 are repeated until an affirmative decision isobtained in step S12, that is, until the stepper motor 20 has beenenergized by four steps. In other words, the steps S8-S12 cause thestepper motor 20 to perform one complete energization cycle (with thephases changing in the following order: C-D, D-A, A-B and B-C) after theaffirmative decision has been obtained in step S6. Then, the steppermotor 20 is placed again in the phase C-D after the motor windings havebeen energized four steps. Step S12 is followed by step S13 in which thesecond timer is again started to allow 40 msecs to elapse with the phaseC-D maintained, before the CPU 70 goes back to a main control routine.

While the step S9 is executed for the first time after an affirmativedecision is given in step S6, the rotor teeth 19a of the stepper motor20 are placed in a phase C-D position, i.e., at a position at which therotor teeth are offset the same distances from the stator teeth 21a, 21aon the adjacent pole pieces 21 on which the windings C and D are wound(FIG. 3). In FIG. 6, the true phase C-D position corresponding to thehome position of the stepper motor 20 is indicated at X, and the wrongphase C-D position neighbouring the home position is indicated at Y.While the phase C-D is maintained for 40 msec during the first executionof step S9, the rotor 19 is placed either in the true phase C-D positionX, or in the wrong phase C-D position Y. In either case, the rotor 19oscillates around the phase C-D position X or Y, with a relatively largeamplitude in the initial portion of the 40 msec time span which iscounted by the second timer in step S9. However, the oscillation of therotor 19 is finally settled, and the rotor 19 is brought to the phaseC-D position X or Y at the end of the 40 msec holding of the phase C-D.

In the case where the rotor 19 is placed in the wrong phase C-D positionY at the end of the 40-msec phase C-D maintenance in the first executionof step S9, the rotor 19 is then incremented in the forward directionthree steps with the phases D-A, A-B and B-C being sequentiallyestablished as indicated in FIG. 6, as a result of repeated execution ofsteps S9-S12, which cooperates with the first C-D phase energization toconstitute one energization cycle of the motor 20. Then, the rotor 19 isagain brought to the true phase C-D position X, which is maintained forthe 40 msec period in step S13. Namely, the rotor 19 is maintained inthe true phase C-D corresponding to the home position of the motor 20.If the rotor 19 is placed in the true phase C-D position X, the rotor 19is held in the phase C-D position X during the first execution of stepS9. This phase C-D position X may or may not be maintained during thenext phase D-A energization. Namely, the rotor 19 may possibly be movedto the phase D-A position following the phase C-D position, as indicatedin solid line in FIG. 6, due to a slight pivotal movement of the detentarm 52 which may arise from an elastic deformation of the resilientmember 62. In either instance, when the next phase A-B energization iseffected, the mechanical blocking of the gear 24 does not permit therotor 19 to be rotated to the following phase A-B position, whereby therotor 19 will be rotated a slight angle in the reverse direction, andbrought to the wrong phase A-B position as indicated in FIG. 6.Subsequently, the phase B-C and phase C-D energizations are effected toincrement the rotor 19 two steps in the forward direction, whereby therotor 19 is finally placed in the true phase C-D position X, that is, inthe predetermined home position.

If the amplitudie of the rotor 19 of the stepper motor 20 after themechanical blocking is large to such an extent that the rotor 19 jumpsin the reverse direction to another wrong phase C-D positionneighbouring the wrong phase C-D position Y, the rotor 19 may berestored back to the true phase C-D position X (home position) bysetting the counter to "8", "12" or other multiples of "4" in step S8.

On the other hand, if the amplitude of the oscillating movement of therotor 19 after the mechanical blocking is considerably small, and therotor 19 has a tendency that the rotor 19 jumps to the phase B-Cposition just before the phase C-D position X, as indicated in solidline in FIG. 7, or to the preceding phase A-B position as indicated inFIG. 8, a value "1" or "2" is set in the counter in step S8 so that thewindings of the stepper motor 20 are energized only one step (phase B-Cto phase C-D) or two steps (phase A-B to phase B-C, and to phase C-D).

Although it has been assumed for the sake of easy understanding that onestep energization of the stepper motor 20 results in one step indexingof the print wheel 28 to change the typing elements 32 from one toanother, the one step indexing of the print wheel 28 corresponds to aplurality of energization steps of the stepper motor 20. In this case,the value to be set in the coutner in step S1 of the motor homingroutine of FIG. 5 is equal to:

    (N+a)×n

where,

N: number of the typing elements 32,

a: positive integer (including zero) which corresponds to a desiredangle of rotation of the print wheel 20 to be added to one fullrevolution given by N,

n: number of steps of the stepper motor that gives one indexing motionof the print wheel 28 corresponding to a spacing between the adjacenttyping elements 32.

In this case, too, the stepper motor 20 is energized one step in step S2of FIG. 5.

Although the embodiment of FIGS. 5-6, and the modified embodiments ofFIGS. 7 and 8 are adapted such that the stepper motor 20 is operated inthe simultaneous two-phase energization mode, the principle of thepresent invention is applicable to a stepper motor which is operated ina single phase mode, or an alternate single and two phase mode. In thesingle phase mode, the stator windings A, B, C and D are sequentiallyenergized. In the alternate single and two phase mode, the firstenergization occurs on the winding A, the second energization occuringon the windings A and B, the third energization occuring on the windingB, the fourth energization occuring on the winding B and C, and so on,for example.

While the present invention has been described in its preferredembodiments, it is to be understood that the invention is not limitedthereto, but the invention may be embodied with various changes,modifications and improvements which may occur to those skilled in theart, without departing from the spirit and scope of the inventiondefined in the following claims.

What is claimed is:
 1. A method of bringing a movable element of astepper motor to a predetermined home position, comprising the stepsof:moving said movable member in one direction by repeating anenergization cycle wherein windings for sequentially establishing aplurality of energization phases of said motor are energized in apredetermined sequence at a predetermined energization interval from oneenergization phase to another; mechanically blocking said movableelement from further moving from said home position in said onedirection; after mechanically blocking said movable element, energizingat least one of said windings which corresponds to at least one of saidenergization phases that precedes, in said predetermined sequence,another of said energization phases which corresponds to saidpredetermined home position of said movable element, said at least oneof said windings being energized for a time span longer than saidpredetermined energization interval of said energization cycle, for eachof said at least one energization phase preceding said anotherenergization phase; and maintaining said another energization phase andthereby holding said movable element at said predetermined homeposition.
 2. A method according to claim 1, wherein said step ofenergizing at least one of said windings comprises energizing thewindings in said predetermined sequence to perform at least oneenergization cycle of said motor.
 3. A method according to claim 1,wherein said movable element consists of a rotor rotatable about an axisthereof.
 4. A method according to claim 1, wherein said movable elementof said stepper motor is a sliding member which is linearly slidable. 5.A combination comprising:a stepping motor including windings forsequentially establishing a plurality of energization phases, and amovable element which is movable in opposite directions; a motor drivercircuit connected to said windings, for moving said movable element inone of said opposite directions, by repeating an energization cycle inwhich said windings are energized in a predetermined sequence at apredetermined energization interval from one energization phase toanother; a mechanical blocking means for mechanically blocking saidmovable element from further moving from a predetermined home positionin said one direction; and homing control means operable after saidmovable element has been mechanically blocked by said mechanicalblocking means, for activating said motor driver circuit to energize atleast one of said windings which corresponds to at least one of saidenergization phases that precedes, in said predetermined sequence,another of said energization phases which corresponds to saidpredetermined home position of said movable element, said at least oneof said windings being energized for a time span longer than saidpredetermined energization interval of said energization cycle, for eachof said at least one energization phase preceding said anotherenergization phase, said homing control means then activating said motordriver circuit to maintain said another energization phase, and therebyhold said movable element at said predetermined home position.
 6. Acombination according to claim 5, wherein said homing control meansactivates said motor drive circuit to energize the windings in saidpredetermined sequence, in order to perform at least one energizationcycle of said motor.
 7. A printer for printing on a recording medium,comprising:support means for supporting said recording medium; a printwheel rotatable about an axis thereof, and having a multiplicity ofradial arms which bear at free ends thereof respective typing elements;a rotary stepping motor including a rotor rotatable in oppositedirections and operatively connected to said print wheel, and windingsfor sequentially establishing a plurality of energization phases; amotor driver circuit connected to said windings, for repeatedlyperforming an energization cycle in which said windings are energized ina predetermined sequence at a predetermined energization interval fromone energization phase to another, whereby said rotor is rotated tobring selected one of said typing elements into a printing position; ahammer operable to impact said selected one typing element in saidprinting position against said recording medium supported by saidsupport means; a mechanical blocking means for mechanically blockingsaid print wheel from further rotating from a predetermined homeposition in one of said opposite directions; and homing control meansoperable after said movable element has been mechanically blocked bysaid mechanical blocking means, for activating said motor driver circuitto energize at least one of said windings which corresponds to at leastone of said energization phases that precedes, in said predeterminedsequence, another of said energization phases which corresponds to saidpredetermined home position of said movable element, said at least oneof said windings being energized for a time span longer than saidpredetermined energization interval of said energization cycle, for eachof said at least one energization phase preceding said anotherenergization phase, said homing control means then causing said motordriver circuit to maintain said another energization phase, and therebyhold said rotor at said predetermined home position.
 8. A printeraccording to claim 7, wherein said time span for which said at least onewinding is energized for each of said at least one energization phase islonger than a time duration necessary for substantially settling arotary oscillating movement of said print wheel during a mechanicalblocking of said print wheel by said mechanical blocking means.
 9. Aprinter according to claim 7, further comprising:a carriage carryingthereon said stepping motor and said print wheel, and movable in adirection parallel to said recording medium supported by said supportmeans; and a wheel holder connected to said rotor of said stepping motorrotatably about an axis thereof, and holding said print wheel, andwherein said mechanical blocking means includes: a detent arm attachedto said carriage pivotally about an axis parallel to said axis ofrotation of said wheel holder, said detent arm having a first stopmember; a second stop member provided on said wheel holder, such thatsaid second stop member is engageable with said first stop member duringrotation of said wheel holder, for mechanically blocking said wheelholder, and thereby blocking said print wheel from further rotating awayfrom said predetermined home position; and an arm actuator for pivotallymoving said detent arm between an operative position in which said firststop member is engageable with said second stop member, and aninoperative position wherein said first stop member is out of a path ofsaid second stop member.
 10. A printer according to claim 9 wherein saidarm actuator comprises:biasing means for biasing said detent arm towardsaid inoperative position; and a stationary actuator member which isengageable with said detent arm, to pivot said detent arm to saidoperative position against the biasing action of said biasing means,when said carriage is moved to a predetermined position.
 11. A printeraccording to claim 9, wherein said detent arm is provided with aresilient member fixed thereto, said resilient member being elasticallydeformable to permit said carriage to move to said predeterminedposition, even if said wheel holder is positioned such that said secondstop member abuts on said first stop member during a pivotal movement ofsaid detent arm toward said operative position.
 12. A printer accordingto claim 9, wherein said detent arm is provided with a resilient memberfixed thereto, said resilient member being elastically deformable toabsorb a positional error between said carriage in said predeterminedposition and said stationary actuator member.